Two wire current transmitter responsive to a resistive temperature sensor input signal

ABSTRACT

Circuitry including a resistance sensor is excited by an external power source. A reference voltage is established, the voltage developed across the sensor resistance is compared at an amplifier input with a reference voltage signal and the voltage across a feedback resistor, and the total current drawn through the circuitry adjusts as a function of the sensor resistance to give a balanced amplifier input. Total current may be made a linear function of sensor resistance or a non-linear function by proper selection of certain resistors.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a Division of my copending application Ser. No.14,748, filed Feb. 24, 1970 for Two Wire Current Transmitter ResponsiveTo A Resistance Sensor Input Signal now U.S. Pat. No. 3,859,594 which inturn is a continuation of my application Ser. No. 661,988, filed Aug.21, 1967 for Remote Measuring System Utilizing Only Two Wires ForSupplying Current To The Sensing Circuitry And Adjusting The CurrentFlow So That It Is Representative Of A Variable Condition, nowabandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to modulation or control of an electrical currentwhich is dependent on the resistance of a sensing element where the twowires which carry power to the sensing circuitry are also used as thesignal current transmission means. The measurement is one in whichdirect current power is supplied to remotely located sensing and currentmodifying circuitry which acts to control the total current flowproportional to a measurand.

2. Description of the Prior Art

The problem of conversion of a measurand (where "measurand" refers tothe quantity or physical variable being measured) to an electricalsignal and subsequent transmission of that signal to recorder andcontrol equipment which may be located some distance away has beenhandled in many ways in the past. In general four-wire systems have beenused where power is supplied via two of the wires and a voltage signalis transmitted via the other two wires. One of the voltage signal leadsmay be common to one of the power leads for some of these systems. Sucha system typically requires use of an amplifier and/or other signalconditioning equipment at the point of measurement in order to supply anaccurate signal representative of the measurand. The advantages of usingthe same two wires for power supply and information transmission haslong been recognized and various designs of transmitting equipment haveevolved. The prior art approach for force transducers where directcurrent power and signals are required has been to supply sufficientcurrent to rebalance the force being measured by current through anelectromagnetic arrangement. A small amount of current is routed througha null sensing circuit and amplifier which in turn controls the maincurrent supply to the force balance coil. Such circuitry is widely usedin industrial pressure measurements and an example is described in U.S.Pat. No. 3,274,833.

For temperature sensing transmitters self-balancing circuits using amotor driven potentiometer have commonly been used. Examples of variousdesigns which have been developed in the past are given in the chaptertitled "Measuring and Transmission Methods" of the book Handbook ofApplied Instrumentation, McGraw-Hill Book Co., 1964, Library of CongressCatalog Card No. 62-21926. Those various designs all employelectro-mechanical elements of one type or another, to achieveself-balancing circuitry operating from a resistance signal orthermocouple signal. In almost all cases a two phase motor is relied onto provide adjustment of a potentiometer or variable condenser toachieve a balance condition. In one example, the current from athermocouple passes through the field of a permanent magnet deflecting abeam against a calibrating spring. Beam deflection is sensed by othercircuitry which supplies a high level, direct current signal suitablefor transmitting to a recorder. A portion of the signal is shuntedthrough a feedback coil which opposes the force caused by thethermocouple current thereby maintaining a balanced condition.

Since these previous designs have all required electromechanical devicesthey have the disadvantages of rather slow response, limited lifeassociated with a loss of resolution where frictional contacts areinvolved, and poor performance under adverse environments such as widevariations in temperature, excessive humidity and dust.

SUMMARY OF THE INVENTION

This invention comprises a resistance network resembling a bridgearrangement including a first resistor which changes resistance inresponse to a measurand and a second resistor which has one endconnected to the output of an amplifier controlled current regulator.The output of the bridge is fed to the amplifier input with the correctpolarity to always insure a balanced condition at the amplifier inputterminals due to opposing signals arising from the first resistor andthe feedback current through the second resistor. The circuit isself-balancing and the total current drawn by the circuit isproportional to the measurand value. In one embodiment linearization ofthe relation between measurand and total current is provided by havingthe feedback current effectively adjust the bridge excitation inaddition to balancing the bridge.

The resulting circuit does not require electromechanical devices such asrotary or linear motors and is free of the disadvantages of suchdevices. Conversion or transduction of a measurand signal into a directcurrent signal is accurately accomplished with a minimum of componentsresulting in a high performance transmitter having long life and highreliability.

It is therefore a primary object of this invention to provide anonmechanical self-balancing circuit responsive to a measurand where thetotal current drawn by the circuit is proportional to the measurand.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic showing the basic circuit of the invention helpfulin understanding the invention and the manner in which it is used.

FIG. 2 is a circuit schematic showing details of a preferred embodimentuseful for practicing the invention.

FIG. 3 is a schematic showing an alternate arrangement of some of thebasic circuit components.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to the drawings and the reference notations thereon FIG. 1shows a closed series network 10 of resistors R₁ through R₅ connected inan arrangement which somewhat resembles a conventional bridge. ResistorR₁ is adjacent R₂ and opposite R₃ and the series combination of R₄ andR₅ is opposite R₂ and R₄ being connected to R₁ at signal output terminal11 and R₅ connected to R₃ at junction 12. Signal terminal 13 is betweenR₂ and R₃ and the cathod of Zener diode V_(c) is connected to terminal14 between R₁ and R₂. The symbol V_(c) designates a source of referencevoltage having a voltage level of V_(c) volts. The anode of Zener diodeV_(c) is connected to one end of R₆ at terminal 26 and the other end ofR₆ is connected to junction 12. Total current drawn by the network 10 isdesignated I_(t) and is shown as being received by line 15 whichconnects the output of a differential input current controller 16 toterminal 14. The total current I_(t) leaves network 10 by line 17 whichconnects junction 18 between R₄ and R₅ to power return terminal 19. Thetotal current I_(t) is supplied to the current controller 16 from inputterminal 20 which is connected to 16 by line 21. The signal terminals 11and 13 connect to current controller 16 by lines 22 and 23 respectively.Controller 16 responds to a voltage difference between lines 22 and 23in such a manner to reduce any such voltage to substantially zero byadjustment of total current I_(t) and there is substantially zerocurrent drawn by lines 22 and 23. A direct current source 24 and a load25 are shown serially connected across terminals 19 and 20.

In operation a change in one or more of the resistors R₁ through R₅ as aresult of a measurand change ("measurand" refers to the quantity orphysical variable being measured such as temperate or strain) gives riseto a voltage signal between lines 22 and 23 which causes the controllerto adjust the current level I_(t) to reduce the voltage signal to zero.The change in current I_(t) is thus a measure of change in resistance ofone or more of the resistors R₁ through R₅ and this change in current ismonitored by load 25 which may be a recorder for example. For anunderstanding of the circuit response assume that R₆ is zero. In thiscase the voltage across terminal 14 and junction 12 is a constant valueV_(c) regardless of the current flow through diode V_(c). Accordinglythe voltage on line 23 is held constant by V_(c) so long as the ratiobetween R₂ and R₃ does not change. Then assuming R₁ is constant and R₄is allowed to vary, the voltage on line 22 will rise with an increase inR₄. In order to have a self balancing response this increase in voltageon line 22 must give rise to an increase in current I_(t) fromcontroller 16. The increase in current will not affect the voltage online 23 (since it is assumed that for this example R₆ is zero) howeverit will have an opposing effect to the voltage on line 22 since it willgive a change in voltage across resistor R₅ of opposite sign to thechange in voltage across R₄ due to the assumed increase in resistance ofR₄. Then to consider the effects of a finite value of R₆ assume R₆ to beof value such that the voltage drop across R₆ is small compared toV_(c). Then an increased current through the series combination of V_(c)and R₆, caused by an increase in R₄ for example, raises the voltage online 23 by an amount proportional to the increase in voltage across R₆.The same percent increase would also be felt as a part of the voltagechange on line 22. Since the voltage on line 22 is assumed to be greaterthan that on line 23 due to an increase in R₄ the effect of theincreased voltage across R₆ is to require still more current I_(t) toflow to reduce the voltage difference to zero between lines 22 and 23.Since R₆ introduces a correction or adjustment based upon the change incurrent I_(t) its effect is a higher order effect and it may be used assuch to selectively shape the relation between current I_(t) andresistor R₄ for example. Analysis of the network 10 gives the followingexpression between I_(t) and the various resistors shown: ##EQU1##

It may be noted that if R₆ is zero the relation between I_(t) and R₄ islinear and further that if R₆ is not zero the current I_(t) will respondto positive changes in R₄ in an increasingly sensitive manner. It mayalso be noted that an increasing R₂ would result in I_(t) increasing ata less than linear rate if R₆ is finite. An increasing R₁ or R₃ wouldgive a decreasing current I_(t) and the rate of decrease would reducewhether or not R₆ was zero however the magnitude of reduction dependssomewhat on R₆.

As a further example consider linearization of the relation betweenI_(t) and temperature when a platinum resistance thermometer is used asthe measurand sensing instrument. Picking R₄ as the thermometer therelation between resistance and temperature is

    R.sub.4 =R.sub.0 (1+αT-βT.sup.2)                2.

for temperatures 0° Celsius and higher, where R₀ is the resistance at 0°C., T is temperature in degrees Celsius and α and β are constants.Substituting expression (2) into expression (1), expanding terms andsolving for the condition causing disappearance of terms involving T² inthe numerator gives ##EQU2##

Accordingly it is only necessary to satisfy expression (3) to achieve alinear relation between total current I_(t) and temperature when aresistance thermometer such as platinum is used as R₄. Similar analysismay be made for the case where R₂ is a platinum thermometer. If the rateof resistance change as a function of temperature increases, as it doesfor a nickel wire thermometer for example, the element may be shunted bya constant resistance to linearize the response and it may then be usedin place of R₄ with a zero value for R₆. A thermistor or thermistornetwork involving a series-shunt combination of resistors having anegative change of resistance with temperature would preferably be usedin place of R₁ or R₃.

In some cases it is desirable to have more than one resistor respond tothe measurand. Resistance strain gage measurements commonly employ atleast two resistors, one increasing with strain and one decreasing, andthese would be preferably located in adjacent positions of network 10.If a temperature difference as sensed by two thermometers having likecharacteristics is to be measured these resistance thermometers wouldalso be located in adjacent portions of network 10, for example in placeof R₄ and R₃. A range or span adjustment can be conveniently made byadjustment of magnitude of R₅ and zero may be adjusted by R₃ forexample.

In some instances it is desirable to use the circuitry shown forconverting a low level voltage signal such as a thermocouple output to acontrolled current signal. A thermocouple or other voltage signal may beintroduced in series with one of the resistors R₁ through R₅ or byconnection in series with, or across, current controller signal lines 22and 23. Resistor R₄ may be selected to be temperature responsive also sothat it serves as a reference junction compensation for a thermocouplehaving its reference junction adjacent R₄. In all such cases thedifferential input current controller responds to a voltage signalacross lines 22 and 23 in such a manner to reduce that voltage to zeroand the resulting current drawn by the circuit and available formeasurement across load 25 bears a predetermined relation to the voltagesignal and its origin.

A detailed schematic of a differential input controller together withnetwork resistors R₁ through R₆ and reference voltage V_(c) is shown inFIG. 2. The system of FIG. 2 is a carrier amplifier type controllergiving a high degree of freedom in choice of direct current voltagelevels throughout the circuit. While a "straight" DC or non-carrier typecontroller may be used the carrier amplifier type controller generallygives overall higher performance than would be available with a directcurrent coupled amplifier.

The network resistors R₁ through R₆ and reference voltage V_(c) areconnected in FIG. 2 in the same arrangement of FIG. 1 however the mainsupply of controller feedback current is now delivered to the network atterminal 26 between R₆ and the anode of Zener diode V_(c). The circuitis arranged to receive direct current power at terminal 20 whichconnects to a current controller stage designated generally at 30. Thecurrent is controlled by stage 30 in response to a signal from ademodulator 40 which in turn connects to the output of a differentialamplifier 50. Amplifier 50 responds to the signals across networkterminals 11 and 13 which are coupled to amplifier 50 by a modulator 60.Modulator 60 and demodulator 40 are synchronously driven by amultivibrator 70 which is a square wave, symmetrical, free-running typemultivibrator. The modulator 60 and demodulator 40 may be referred to aschoppers and the multivibrator is a specific example of a chopper drivergenerating a wave form commonly referred to as chopper drive. The totalcurrent drawn by the circuit is effective in obtaining a zero voltagedifference across network terminals 11 and 13 in the same manner asdescribed with reference to FIG. 1 and consequently the current I_(t)which would be measured by a serially connected load as was shown inFIG. 1 is accurately described by expression (1) when the circuit isoperating in a balanced condition.

Current controller 30 includes a pair of transistors 31 and 32 havingtheir collectors connected to input power terminal 20. These transistorsform a Darlington amplifier since the emitter of 32 connects throughresistor 33 to power line 81 which line is the main source of power fromthe other circuit elements. Controller 30 also includes transistor 34and Zener diode 35 which operate to give a substantially constantcurrent in the base to emitter circuits of 31 and 32 even though theinput power supplied to terminal 20 may vary considerably in voltagelevel. The base of transistor 31 connects to the collector of 34 andconnects through resistor 36 to the junctions between the cathode ofZener diode 35 and resistor 37. The other end of resistor 37 connects toinput terminal 20. The emitter of transistor 34 and anode of diode 35connect to line 81 and the base of transistor 34 connects to the outputof demodulator 40 through resistor 41. A capacitor 38 is connectedbetween the base of transistor 34 and the emitter of transistor 32 inorder to shunt any high frequency components that may appear attransistor 34.

The demodulator 40 includes an N-channel field effect transistor 42connected in series with a P-channel field effect transistor 43 atjunction 44. The source of transistor 42 connects to line 81 and thesource of transistor 43 connects to resistor 41 at demodulator outputterminal 45. Resistors are respectively connected from gate to source oftransistors 42 and 43 and the transistors are alternately madeconducting and non-conducting by a capacitively coupled output signal online 83 which is connected to the output of multivibrator 70. A positivesignal on line 83 cuts off transistor 43 and turns on transistor 42thereby effectively referencing the output of amplifier 50 to power line81. The alternate negative signal on line 83 cuts off 42 and turns on 43thereby coupling the output of amplifier 50 to the output terminal 45 ofdemodulator 40.

Amplifier 50 is a direct current integrated circuit differentialamplifier having its output capacitively coupled to resistor 53 andthence to junction 44. Input power is obtained from line 81 and powerreturn is to line 82. Amplifier signal input terminals 51 and 52 arecapacitively coupled to modulator output terminals 61 and 62respectively. The series combination of resistor 54, capacitor 55 andresistor 56 is connected between input terminals 51 and 52. The outputof amplifier 50 is D.C. connected to the junction between resistor 54and capacitor 55 by means of degenerative feedback resistor 57. Thisdegenerative feedback of direct current signals insures that unwantederror signals such as thermoelectric potentials at the input terminalswill have litle effect on the amplifier control signal. An alternatingvoltage signal from modulator 60 will be amplified independently byamplifier 50 because of the capacitive coupling of both input and outputterminals. The amplifier 50 is operated from a balanced voltage suppliedby lines 81 and 82 which is maintained substantially constant by seriesconnected Zener diodes 84 and 85. These diodes are of the same type andsame voltage breakdown. The junction between the cathode of 85 and theanode of 84 is connected to the junction between capacitor 55 andresistor 56 at the input to amplifier 50 by line 88 thereby maintainingthe input of amplifier 50 midway between the voltage on lines 81 and 82.

Modulator 60 includes field effect transistor 63 which is alternatelymade conducting and non-conducting by the multi-vibrator outputcapacitively coupled to the gate of transistor 63 from line 86. Theoutput connections of transistor 63 connect directly to modulator outputterminals 62 and 61 respectively. These output terminals are resistancecoupled to network output terminals 11 and 13 respectively so that aD.C. output signal arising at terminals 11 and 13 from a networkunbalance is alternately shorted and applied across amplifier terminals51 and 52 at the frequency established by multivibrator 70.

Multivibrator 70 receives power from line 81 and has a power return toline 82. The multivibrator includes a pair of transistors oscillating ina continuous manner and, as shown, is of conventional design whichrequires no elaboration.

The basic operation of the circuit of FIG. 2 is similar to thedescription given in reference to FIG. 1. Amplifier 50 and currentcontroller 30 function to maintain zero voltage between networkterminals 11 and 13 and the total current drawn by the circuit isrelated to the network resistors R₁ through R₆ and reference voltageV_(c) by expression (1). Amplifier 50 and multivibrator 70 are eachpowered between lines 81 and 82 which are maintained at substantiallyconstant voltage by Zener diodes 84 and 85. Consequently the currentcomponent drawn by these elements is active in the network balance sincethe current return is from line 82 through linearizing resistor R₆ andcurrent feedback resistor R₅ to output terminal 19. This currentcomponent is typically small and relatively constant and the main signalcurrent is developed by virtue of a network unbalance resulting in achange in current through Zener diodes 84 and 85 and thence over line 82through resistors R₆ and R₅ to output terminal 19. Resistor 87 couplespower from line 81 to the cathode of reference source V_(c) which inturn supplies the network in the manner described with reference toFIG. 1. Resistor 87 substantially blocks the balancing current suppliedfrom controller 30 from passing through reference element V_(c) therebyminimizing any change in V_(c) which might otherwise be caused byrelatively large changes in current through V_(c).

As an example of operability it was desired to deliver an output currentvarying from 10 to 50 milliamps for a temperature change from 0° C. to100° C. as measured by a platinum resistance thermometer. A sensorresistance of nominally 100 ohms at 0° C. was selected and was simulatedby a manually variable resistor substituted for R₄. Other networkresistor values were 1.008 ohms for R₅, 2.26 ohms for R₆, 90.78 ohms forR₃ and 6187.2 ohms for each of R₁ and R₂. Reference source V_(c) was aIN-827 Zener diode having a nominal voltage of 6.2 volts and Zenerdiodes 84 and 85 were type IN-4739 controlling at about 8.4 volts each.Resistor 87 was 1850 ohms and the input voltage from the D.C. source 24was approximately 60 volts. Amplifier 50 was a type 709C direct currentoperational amplifier manufactured by Fairchild Semi-conductor, MountainView, California and the other components were of size and type tomaintain the various circuit elements within their design ranges ofoperation.

When resistance R₄ was varied to correspond to the well known resistancechange of platinum with temperature the results of Table 1 wereobtained.

                  TABLE 1                                                         ______________________________________                                        Simulated                                                                     Temperature    Resistance R.sub.4                                                                          Current I.sub.t                                  ______________________________________                                         0 deg. C.     100.00 ohms   10.000 ma                                        25 deg. C.     109.92 ohms   19.999 ma                                        50 deg. C.     119.77 ohms   30.000 ma                                        75 deg. C.     129.55 ohms   40.004 ma                                        100 deg. C.    139.25 ohms   50.001 ma                                        ______________________________________                                    

The results in Table 1 are one example of the close agreement which isobtained between a measurand and output current for the circuits hereindisclosed. Although the example was for a current range of 10-50 ma fora 100 ohm temperature sensor and a 100° C. range it is apparent that thecircuits are suitable for operation over a wide range of variables andthe current range achieved may also be selected over a wide range whileusing the circuits which are described and illustrated herein.

An alternate schematic of the general network 10 shown in FIG. 1 isshown in FIG. 3. The network of FIG. 3 is substantially equivalent tonetwork 10 and the operation follows the same formula relating totalcurrent, reference source V_(c) and the resistors making up the network.The network of FIG. 3 was derived from 10 by transformation of the "wye"circuit comprising R₃, R₅, and R₆ of FIG. 1 to the "delta" circuitcomprising resistors, R₇, R₈, and R₉ of FIG. 3. Expression (1) may alsobe applied to the network arrangement of FIG. 3 where the followingtransformations apply: ##EQU3##

The network arrangement of FIG. 3 may be substituted directly into thecircuit of FIG. 1 or the circuit of FIG. 2 by connecting terminals 11,13, 14 and 19 to the terminals of like numbers in FIG. 1 or FIG. 2 anddisconnecting the corresponding networks shown in those Figures. Thecurrent derived from differential input current controller 16 may beapplied directly to terminal 14 as shown in FIG. 1 but in the preferredembodiment the major portion of the controlled current is delivered tothe network at terminal 26 as was shown in the circuit description ofFIG. 2.

What is claimed is:
 1. A resistance-to-current converter systemcomprising:a. a sensing resistor responsive to a variable physicalcondition to be indicated, b. a bridge including said resistor, saidbridge having input and output junctions and being balanced only whensaid resistor has a predetermined value, whereby the bridge voltagedeveloped at said output junctions is a function of the deviation of theresistor from said predetermined value as a result of a change in saidphysical condition, .Iadd.said bridge further comprising voltagereference means connected across said input junctions to stabilize thevoltage thereacross, .Iaddend. c. a D-C amplifier whose input circuit isconnected to the output junctions of said bridge whereby the impedancein the output circuit of the amplifier varies as a function of saidbridge voltage, d. a direct-voltage source providing an operatingvoltage both for said bridge and said amplifier, e. a load connected inseries with said source and having an output current flow therethroughresulting from variations in said resistor and hence indicative of saidcondition, f. a voltage regulator, g. a pair of wires connecting saidseries-connected source and load through said regulator across both theoutput circuit of said amplifier and the input junctions of said bridge,whereby the operating voltage of said amplifier is stabilized despitevariations in said output current flow through said load resulting fromvariations in the value of said resistor, h. a feedback resistorconnected in series with said load to produce a feedback voltage, and i.means to feed said feedback voltage into the arm of said bridgecontaining said sensing resistor in series opposition to the voltageproduced by the sensing resistor to rebalance the bridge, whereby theoutput current is accurately proportional to changes in said resistor.2. A converter system as set forth in claim 1.[., further including.]..Iadd.wherein said voltage reference means comprises .Iaddend.a Zenerdiode connected across said input junctions to stabilize the voltagethereacross.
 3. A system as set forth in claim 1 wherein said regulatoris constituted by a transistor having its emitter and collector inseries with one of said wires, a reference voltage being applied to thebase of said transistor.
 4. A system as set forth in claim 1, whereinsaid resistor is a temperature-responsive element, and said load is aread-out device to indicate the temperature sensed by said element.
 5. Asystem as set forth in claim 4 wherein said resistor is made ofplatinum.
 6. A system as set forth in claim 4 wherein said resistor andamplifier are located in a first location, whereas the load and D-Csource are located at a remote second location.
 7. An electrical currenttransmitting system comprising a pair of current terminals adapted forconnection to a load and power source that are in series; a resistancenetwork means connected in series with said current terminals to bepowered and excited solely therethrough and including a first resistorwhich varies in resistance value as a function of temperature, forproviding a first variable D.C. voltage signal which .Iadd.depends on areference voltage and .Iaddend.varies as a function of the temperatureto be measured; .Iadd.means connected to said resistance network meansto generate said reference voltage; .Iaddend.a feedback resistance meansconnected to said resistance network means; means to be powered solelythrough said terminals connected to the feedback resistance means and tothe current terminals to establish a current flow path between saidterminals and to provide a known portion of current flowing between saidterminals through said feedback resistance means to provide a secondD.C. voltage signal which is a function of the current through saidcurrent terminals including an amplifier means having a voltage inputportion and an output portion providing an output signal which is afunction of the voltage at said voltage input portion, and currentcontrol and adjustment means coupled to said output portion to receivethe output signal and to adjust the current in said current flow pathbetween said terminals as a function of said output signal; and means tocouple said resistance network means and said feedback resistance meansto said voltage input portion to provide a voltage at said input portionof said amplifier means which is a function of said first and secondvoltage signals.
 8. The combination of claim 7 further characterized inthat .[.a.]. .Iadd.said .Iaddend.voltage reference means is connected tosaid resistance network means in series with said feedback resistancemeans between said terminals to provide .[.a.]. .Iadd.said.Iaddend.reference voltage for said resistance network means.
 9. Thecombination of claim 8 wherein said voltage reference means includes afixed resistance and a semiconductor reference element in series withsaid fixed resistance, the series combination of said reference elementand fixed resistance being connected across said first resistor so thatan increase in excitation voltage across said first resistor is providedas a result of an increased current through said voltage referencemeans.
 10. The combination as specified in claim 7 wherein said feedbackresistance means includes a second resistor, said resistance networkmeans further includes third and fourth resistors, said first, second,third and fourth resistors being connected in a symmetrical bridgearrangement having one end of said first resistor connected to one endof said third resistor to provide a first voltage terminal, and one endof said second resistor being connected to one end of said fourthresistor to provide a second voltage terminal, and wherein the otherends of said first and second resistors are connected to one of saidcurrent terminals and the other ends of said third and fourth resistorsare connected together to the other current terminal, said feedbackresistance means further including fifth and sixth resistors, and.[.a.]. .Iadd.said .Iaddend.voltage reference means for providing areference voltage for said resistance network means including a voltagereference element having one end connected to the junction of said thirdand fourth resistors and the other end connected through said fifthresistor to .[.the other.]. .Iadd.said one .Iaddend.of said currentterminals and through said sixth resistor to said second voltageterminal.
 11. The current transmitting system of claim 7 wherein saidfeedback resistance means comprises a second resistor, said resistancenetwork means further including third, fourth and fifth resistors, allof said resistors being connected in a closed series loop having thefirst resistor connected between the second and fourth resistors and thethird resistor connected between the second and fifth resistors.Iadd.and the fourth and fifth resistors connected together.Iaddend.,.[.and.]. .Iadd.said voltage reference means comprises .Iaddend.avoltage reference element.Iadd., .Iaddend.and a sixth resistor connectedin series .Iadd.with said voltage reference element .Iaddend.between thejunction of said fourth and fifth resistors and the junction of saidsecond and third resistors, one of said current terminals beingconnected to the junction of the fourth and fifth resistors to carry afirst portion of the current to said resistance network means, and thejunction of the voltage reference element and the sixth resistor beingconnected to said one current terminal to carry a second portion of thecurrent from said one terminal to the resistance network means, saidlast-mentioned junction being connected through portions of saidresistance network means to the other .Iadd.of said .Iaddend.currentterminal. .Iadd.
 12. An electrical current transmitting systemcomprising:a load; a power source connected in series with said load andserving as the sole source of power for said electrical currenttransmitting system; a first resistor which varies in resistance valueas a function of a physical condition; a bridge including said firstresistor and having input and output junctions and being balanced onlywhen said first resistor has a predetermined value; voltage referencemeans coupled across said input junctions to stabilize the voltagethereacross; a direct current amplifier having inputs connected to saidoutput junctions whereby the amplifier output signal is controlled bythe signals at said output junctions; a voltage regulator coupled tosaid input junctions and responsive to said amplifier output signal; apair of wires connecting the series combination of said load and saidpower source to said voltage regulator and to said bridge; a feedbackresistor connected in series with said load to produce a feedbackvoltage; and means to couple said feedback voltage into the arm of saidbridge containing said first resistor in series opposition to thevoltage produced by said first resistor whereby the bridge iseffectively balanced for varying values of said first resistor..Iaddend.